For many years, one of the most popular explosives for use in mining operations has been ammonium nitrate fuel oil (“ANFO”), which is formed of porous ammonium nitrate prills and diesel oil that is situated within the voids of the porous ammonium nitrate prills. In mining operations, the use of ANFO typically involves drilling a blast hole in the earth that is being mined. After a blast hole has been drilled, one or more initiators and ANFO are loaded into the blast hole. The ANFO is then detonated with the result being that the earth that is being mined is fractured in a manner that facilitates the removal of the earth by machinery for further processing. Typically, an array of blast holes is established and the ANFO established in the blast holes is detonated simultaneously or in a sequence that is designed to produce desired blast characteristics.
A drawback associated with ANFO is that it is highly absorbent and the absorption of water reduces the explosive capability of the ANFO. Consequently, ANFO is typically not used in situations in which water in a blast hole is likely to adversely affect the ANFO. The water can be water that is present in the blast hole when the ANFO is loaded and/or water that migrates into the blast hole after the ANFO has been loaded and before the ANFO is detonated. Due to ANFO's highly absorbent nature, ANFO is suitable for use in applications in which water is unlikely to meaningfully reduce the effectiveness of the ANFO. Additionally, when Ammonium Nitrate is mixed with Fuel Oil to form ANFO, the resulting composition is dry to the touch and, as a consequence, commonly referred to as a “Dry Mix” explosive.
To address the “wet blast hole” situation, a water resistant ANFO, commonly referred to as Heavy ANFO, was developed. Heavy ANFO is comprised of an emulsion explosive in combination with ANFO and/or ammonium nitrate prills. An emulsion explosive contains oxidizers that are dissolved in water droplets that are surrounded by an immiscible fuel, such as oil. Due to Heavy ANFO's water resistance, it is suitable for use in applications in which the composition will be exposed or is likely to be exposed to water. Additionally, Heavy ANFO is wet to the touch due to the interstices of the ammonium nitrated being filled within the ammonium nitrate with the emulsion explosive. Hence, Heavy ANFO is sometimes referred to as “wet mix” explosive.
In addition to water resistance, a given volume of Heavy ANFO has greater explosive energy than an equal volume of ANFO. In a mining situation, the greater explosive energy of Heavy ANFO per unit volume typically means that fewer blast holes need to be drilled to blast a particular area relative to the number of blast holes that need to be drilled when ANFO is used. Further, for a given explosive energy, the cost to produce Heavy ANFO is less than the cost to produce ANFO in many situations. Due to the increased explosive energy and reduced cost, Heavy ANFO is also utilized in dry blast hole situations.
A problem associated with Heavy ANFO is that the density of the Heavy ANFO in a blast hole increases with increasing depth in the blast hole. Stated differently, there is a density gradient for the Heavy ANFO in a blast hole. As the density of Heavy ANFO increases, the sensitivity of the Heavy ANFO decreases. Sensitivity is a measure of the amount of energy needed to reliably detonate an explosive. So, a more sensitive explosive requires less energy to detonate than a less sensitive explosive. Consequently, in a blast hole, the Heavy ANFO towards the bottom of the blast hole may be much less sensitive than the Heavy ANFO towards the top of the blast hole. This sensitivity gradient can produce asymmetrical or uneven detonations that fail to have the desired blast characteristics.
To address the density problem in Heavy ANFO, a number of solutions have been proposed. For instance, inert bulking agents in the form of certain agricultural waste products have been added to the Heavy ANFO. However, the agricultural waste products presently known to be used to reduce the density also reduce the sensitivity of the explosive, in many cases, to the point at which the explosive is ineffective. Another approach replaces some or all of the fuel oil in the Heavy ANFO with polystyrene. However, this approach also reduces the sensitivity of the explosive and is generally cost prohibitive. Yet another approach uses micro-balloons and/or perlite, which are known sensitizing agents, to decrease the density of the Heavy ANFO. This approach is not presently considered cost effective. A further approach uses chemical gassing to reduce the density. The drawback of this approach is that the density cannot be effectively controlled over a wide density range.